1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices, and more particularly, to a method of performing salicide processes on MOS transistors.
2. Description of the Prior Art
Field effect transistors are important electronic devices in the fabrication of integrated circuits, and as the size of the semiconductor device becomes smaller and smaller, the fabrication of the transistors also improves and is constantly enhanced for fabricating transistors with smaller sizes and higher quality.
In the conventional method of fabricating transistors, a gate structure is first formed on a substrate, and a lightly doped drain (LDD) is formed on the two corresponding sides of the gate structure. Next, a spacer is formed on the sidewall of the gate structure and an ion implantation process is performed to form a source/drain region within the substrate by utilizing the gate structure and spacer as a mask. In order to incorporate the gate, source, and drain into the circuit, contact plugs are often utilized for interconnection purposes, in which the contact plugs are composed of conducting metals such as tungsten and copper. Nevertheless, the interconnection between the contact plugs and the silicon material of the gate structure and the source/drain region is usually poor, hence a silicide material is often formed over the surface of the gate structure and the source/drain region to improve the ohmic contact between the contact plugs and the gate structure and the source/drain region. Today, the process known as self-aligned silicide (salicide) process has been widely utilized to fabricate silicide materials, in which a source/drain region is first formed, a metal layer comprised of cobalt, titanium, or nickel is disposed on the source/drain region and the gate structure, and a rapid thermal process (RTP) is performed to react the metal layer with the silicon contained within the gate structure and the source/drain region to form a metal silicide for reducing the sheet resistance of the source/drain region.
However, when the silicides are being formed, the atoms within the metal layer will diffuse into the substrate and deplete the silicon within the source/drain region, thereby damaging the original lattice structure of the source/drain region and causing the PN junction between the source/drain region and the silicon substrate to react with the silicon contained within the source/drain region as a result of an overly short distance between the PN junction and the silicide layer. Ultimately, the problems become much worse in the design of ultra shallow junctions (USJ) as the silicides often come in contact directly with the substrate and result in failure of the device.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 are perspective diagrams showing the fabrication of a conventional field effect transistor. As shown in FIG. 1, a gate structure 106 having a gate dielectric layer 102 and a gate electrode 104 is first formed on a substrate 100. Next, an ion implantation process is performed to form a lightly doped drain 110 in the substrate 100. Next, a liner 107 and a spacer 108 are formed on the sidewall of the gate structure 106 and another ion implantation is performed to form a source/drain region 112 in the substrate 100. Next, a sputtering process is performed to form a metal layer 114 over the surface of the gate electrode 104, the spacer 108, and the substrate 100. Subsequently, as shown in FIG. 2, a rapid thermal process (RTP) is performed to react the contact area between the metal layer 114 and the gate electrode 104 and the source/drain region 112 into a silicide layer 116.
In order to prevent the short channel effect of the transistors and improve the interconnect resistance of the integrated circuit, the junction depth of the source and drain needs to be effectively reduced for fabricating transistors containing silicides. However, if the thickness of the silicides on the source and drain is decreased while reducing the junction depth of the source and drain, the interconnect resistance and contact resistance may increase simultaneously. On the other hand, if the depth of the silicides is kept constant, the distance between the PN junction of the source/drain region 112 and the silicon substrate and the silicide layer 116 may become overly short and result in junction leakage and a piping effect.